The present invention relates to a quartz glass crucible having a crucible body with an inner surface of quartz glass. Furthermore, the invention relates to a process for manufacturing a quartz glass crucible by providing a crucible body with an inner surface of quartz glass.
Quartz glass crucibles are employed for example for the pulling of monocrystals from a melt according to the Czochralski method. A crucible of this kind and a manufacturing process therefor are described in U.S. Pat. No. 4,528,163. The known crucible comprises a crucible base body of natural quartz glass with a floor joined to an essentially cylindrical side wall. The crucible has an inner layer of transparent synthetically manufactured quartz glass formed by vitrification of a granulate layer of synthetically manufactured SiO2 granulate.
To produce the known quartz glass crucible, natural quartz crystalline granulate is added, in a first process step, into a metallic mold rotating about its central axis, to form an evenly thick outer layer in the shape of a crucible. With continued rotation of the mold, synthetically produced SiO2 powder is then added and it deposits on the inner wall of the outer layer as a granulate layer, and is subsequently melted to form a resistant inner layer firmly joined to the outer layer. In order to avoid formation of crystal seedlings and bubbles in the inner layer, the melting temperature is set high enough so that the added granulate melts completely. The inner surface of the known quartz glass crucible is therefore formed by a smooth amorphous inner layer. A similar process is also known where a crucible-shaped outer layer of crystalline quartz granulate and a granulate of synthetically produced SiO2 powder are fixed on the inner wall of the mold by vacuum action. The outer layer and the granulate layer are then vitrified together.
When a monocrystal is pulled according to the Czochralski method, for example when a silicon monocrystal is pulled from a silicon melt, a seed crystal of predetermined orientation is dipped into the melt and then pulled slowly upward. The seed crystal and the melt rotate in opposite directions. Surface tension between the seed crystal and the melt causes a small amount of the melt to be pulled along with the seed crystal and as the melt slowly cools it forms a continuously growing monocrystal. However, it may happen in the process that the seed crystal breaks off so that the so-called xe2x80x9cstarting processxe2x80x9d needs to be initiated anew. The period before the actual pulling of the monocrystal can last several hours so that the process becomes correspondingly longer. A long starting process is a considerable expense factor. In addition, as the process time increases so does the thermal and chemical stressing of the quartz glass crucible.
The object of the invention is to provide a quartz glass crucible which simplifies the starting process, and an inexpensive process for the manufacture of such a crucible.
As regards the quartz glass crucible, the object is achieved according to the invention on the basis of the crucible described above in that the inner surface is roughened at least in the starting zone area by multiple depressions disposed at a distance of no more than 5 mm from one another.
During the pulling of a crystal the inner surface is in contact with the melt. The starting zone comprises that area of the inner surface of the quartz glass crucible where the surface of the melt is located during the starting process. It is an area extending around the inner wall of the crucible. It has been shown that a melt surface free of vibrations and oscillations is decisive for a successful starting process. Oscillations of the melt surface, as caused by for example the rotation of the melt and of the seed crystal, or by the dipping of the seed crystal, are dampened by the crucible according to the invention. This is so because the roughening of the inner surface in the starting zone causes a stabilization and a calming of the melt surface. Roughening the starting zone provides depressions in the surface which prevent in-phase wetting and non-wetting of the surface. This measure prevents oscillations from developing or increasing or at least reduces their intensity, so that fluctuations of the melt surface are reduced or dampened. In order for the depressions to develop a corresponding effect, a roughness profile in at least the starting zone is selected such that it comprises multiple depressions disposed no more that 5 mm from one another. The depressions may overlap. What is of substance is that the inner surface is roughened in the starting zone. The relative geometric position of the depressions in relation to the height of the remainder of the inner surface is not relevant therefor. The depression may be below the remaining inner surface but also on it or above it, if additional quartz glass layers are deposited on the inner surface. The roughened area need not be limited to the starting zone. It is also not required for the success of the inventions"" teaching that the surface is roughened without gaps in the circumferential starting zone. It is merely of substance that there is in the region of the melt surface a roughened surface with multiple depressions.
The more pronounced the roughness profile in the starting zone the greater the oscillation dampening effect of the roughness. A particularly well tried roughness has depressions disposed at max. 1 mm from one another, preferably at max. 100 xcexcm.
An inner surface of synthetic quartz glass is distinguished by its purity so that little contamination passes into the melt when the quartz glass crucible is used in applications as intended. The inner surface of synthetic quartz glass is usually formed by depositing on a crucible base body an inner layer a few millimeters thick.
Different variants of the structural design of the surface roughness in the starting zone region have been shown to be advantageous and they may be present alternatively or cumulatively, as described below in more detail.
In a first variant of the embodiment the surface of the inner surface has an etched structure. A suitable etched structure is described for example in DE A 197 13 014. Such etched surface has irregular raised structural elements extending between a first higher plane and a second lower plane, where a plurality of structural elements has a substantially flat top surface extending to all sides and is delimited on all sides in facet like manner by substantially flat side surfaces which extend between the first and the second plane and where the average depth of roughness Ra is between 0.1 xcexcm and 10 xcexcm and the size of the projections of the structural elements to the first plane on average ranges between 30 xcexcm and 180 xcexcm. An etching process and an etching solution suitable for producing such etched structure is also known from DE A 197 13 014. This embodiment is distinguished by a particularly durable surface which in addition substantially reduces the above-described oscillation phenomenon in the melt.
In a further variant of the embodiment the inner surface has a repeating surface impression pattern which is characterized by depressed or raised regularly repeating structures. Due to the regularity of the impression pattern this variant of the embodiment is distinguished by a particularly reproducible effect as regards the stabilization of the starting process.
Another preferred variant of the embodiment of the quartz glass crucible is characterized by an inner surface provided with scores. The scores may be made by for example tools, grinding means, the effect of a particle stream or by laser treatment. In this embodiment the inner surface may be particularly finely structured which can facilitate wetting by the melt and can reduce distortions of the melt surface.
In a further variant of the embodiment the inner surface has a bubble layer with open pores. The open pores provide an additional free volume reservoir for the melt so that a movement of the melt surface can be intercepted effectively.
It has been shown to be particularly advantageous if the inner surface outside of the starting zone region has been fire polished. A fire polished, smooth surface is more easily cleaned than a rough surface and is less readily attacked by the melt. In this embodiment the rough surface is limited to what is necessary for an improvement of the starting process.
As regards the process for manufacturing a quartz glass crucible of this kind, the above-described objective is achieved according to the invention on the basis of the process described initially in that the inner surface is roughened in the starting zone region by multiple depressions which are disposed at a distance of no more than 5 mm from one another.
Roughening of the starting zone provides causes stabilization and calming of the melt surface. Depressions are created in the starting zone which prevent in phase wetting and non-wetting of the surface by the melt. This measure prevents oscillations from developing or increasing or at least reduces their intensity so that fluctuations of the melt surface are reduced or dampened. In order for the roughening to develop a corresponding effect, a roughness profile in at least the starting zone is selected such that it comprises multiple depressions disposed no more that 5 mm from one another. A discussion of the position of the starting zone and the effect and design of the roughening may be found in the above explanation of the quartz glass crucible according to the invention.
The more pronounced the roughness profile in the starting zone the greater the oscillation dampening effect of the roughness. Advantageously, a dense roughness profile is selected, with depressions disposed at max. 1 mm from one another, preferably at max. 100 xcexcm.
Several process variants for producing the surface roughness in the starting zone region have been shown to be advantageous and they may be applied alternatively or cumulatively as described below in more detail.
In a first process variant the surface of the inner surface is roughened by etching with an etching solution. A suitable etching process and a suitable etching solution are described for example in DE A 197 13 014. The etching process is simple to implement and it can simultaneously serve to clean the inner surface of the crucible. The etching may also be limited to an area around the starting zone. Etching results in clean inner surfaces having a xe2x80x98hillyxe2x80x99 structure.
In a further process variant the inner surface is softened and a repeating pattern is impressed into the softened surface. The pattern of impressions is characterized by depressed or raised structures which repeat themselves regularly. It may be produced by for example a pattern die or roller. The impression of a pattern is particularly simple during a process stage when the inner surface of is still soft due to heat residue from the heating of the granulate layer. This eliminates the high temperature step that would have been otherwise necessary in order to soften the inner surface.
In another preferred variant of the process scores are made in the inner surface. Such scores are easily produced by for example diamond tools or by loose abrasive means. The inner surface can also be structured by laser treatment and score structures having defined patterns can also be produced. With a view to a simple production of the quartz glass crucible it has been shown to be advantageous to produce the scores by a particle stream. The particle stream may also be a gas or liquid stream charged with abrasives. In this respect a particularly preferred particle stream is one containing frozen CO2 pellets. The CO2 pellets evaporate without any residue so that contamination by for example remnants of abrasives are avoided.
A variant of the process has been shown to be particularly advantageous wherein production of the crucible body comprises pouring SiO2 granulate into a mold with an additive being added to the SiO2 granulate. The said additive reacts when heated and releases a gas thereby producing a pore containing bubble layer in the region of the starting zone. The pore containing bubble layer can be easily limited to the starting zone region by adding the additive component only during the production of the inner surface in that region. The additive component may be for example Si3N4 or AlN which react during heating and release nitrogen containing gases.
In a further variant of the process a rough surface layer is deposited at least in the region of the starting zone. The surface layer may be produced by pouring in SiO2 granulate according to for example the pouring process described above in more detail, and the roughness is created by for example a low vitrification temperature or by additives with a high melting temperature, such as SiC, added to the SiO2 granulate. This variant of the embodiment is particularly inexpensive since the production of a roughened surface layer is easily incorporated into the manufacture of the crucible by the pouring in method.